The present invention relates to an all-wheel drive for a motor vehicle, having a first axle that can be driven continuously by the vehicle engine or its gear shift transmission and an automatically operating limited slip differential system. This differential system brings two vehicle wheels of a second axle into driving connection with the first axle. The differential system has a planetary transmission formed of sun gear wheels, planetary wheels and a planetary carrier that has a drive through ratio which only slightly deviates from 1:1. The system also has a braking device which is applied to the planetary transmission at a high ratio.
A limited slip differential system of the above type is described in German Pat. No. 35 07 490, corresponding to commonly-owned U.S. application Ser. No. 835,381 now U.S. Pat. No. 4,691,593. That planetary transmission comprises an inlet sun gear wheel connected with the driving axle; an outlet sun gear wheel having a slightly smaller number of teeth that is coaxial and adjacent to said inlet gear wheel and is connected with the axle to be driven; and planetary wheels that are simultaneously engaging with both sun gear wheels and are disposed at a planetary carrier. Bearing bolts are mounted at the planetary carrier on which, as the braking members, brake shoes are disposed which act as centrifugal weights when the planetary carrier turns and with their braking surfaces place themselves against a fixed brake drum serving as the counterbraking member. Since the outlet sun gear wheel has one or two teeth less than the inlet sun gear wheel, the planetary carrier that has a gear ratio of, for example, 16:1 to the sun gear wheels, rotates continuously, even when the front axle and the rear axle run synchronously so that the limited slip differential system continuously results in a power loss even if it is slight. In addition, in order to achieve a synchronous running of the front axle and the rear axle, the pertaining angle drives must differ reciprocally with respect to the through drive ratio. For a motor vehicle with front wheel drive, if the inlet sun gear wheel has a number of teeth z=32 and the outlet gear wheel has a number of teeth z=30, the angle drive ratio to the front axle must be 8:32 and to the rear axle 8:30 in order to compensate the through drive ratio.
An objective of the present invention is to provide a limited slip differential system having an inlet sun gear wheel, an outlet sun gear wheel, and simultaneously engaging planetary wheels with a through drive ratio deviating from 1:1, in which a synchronous movement of the front axle and the rear axle occurs when the angle drive ratios are the same; and when there is a slip-free synchronous movement, a power loss in the limited slip differential system is avoided.
This and other objectives are achieved by the present invention by providing in an all-wheel limited slip differential system having an intermediate planetary transmission in the direct through drive, a braking device engageable at a planetary carrier, the braking device including a braking member connected with the planetary carrier in a torsionally fixed manner, and a counterbraking member connected in a torsionally fixed manner with an output shaft that is connected to one of the vehicle axles.
If a braking member that is connected with the planetary carrier interacts with a counterbraking member that, in a torsionally fixed way, is connected with the power train of the front axle and the rear axle, the whole limited slip differential system rotates along with these axles when the axles move synchronously. The planetary carrier, in this case, does not carry out a relative rotation. As a result, no braking moment is generated and therefore there is no power loss. A braking limited slip moment is not produced before the rotational speeds of the front axle and of the rear axle differ because only then will a relative rotation be produced between the braking member and the counterbraking member. For this reason, the angle drive ratios to the front axle and the rear axle may also be the same.
Other objects, advantages and novel features of the Present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.